CN101079688A - A synchronization method in orthogonal frequency division multiplexing system - Google Patents

Abstract

The invention discloses a synchronous realizing method in the orthogonal frequency-division multiplying system, which comprises the following steps: constructing front/synchronous sequence at the transmitting end to frame and transmit; combining the CP information to do one-time synchronization; utilizing front symbol to estimate the integral times and decimal time-domain frequency deviation; compensating the frequency deviation; using front symbol to do secondary time synchronization; proceeding disposal after synchronizing. The invention improves the synchronous precision to modify the system property as well as the received signal-to-noise ratio, which reduces the calculating quantity without adding the burden of system.

Description

Realize synchronous method in a kind of ofdm system

Technical field

The present invention relates to a kind of OFDM (OFDM) system, relate in particular to the synchronous method of realization in a kind of OFDM wireless receiving system, belong to wireless or the wire communication field.

Background technology

Along with the development of Digital Signal Processing and high speed device, OFDM (OFDM) has obtained successful application in systems such as DVB, DSL and WLAN.OFDM is divided into the subchannel of several quadratures to frequency spectrum at frequency domain, and the carrier wave of each subchannel is overlapped, has improved the availability of frequency spectrum.Because therefore the bandwidth relative narrower of each subchannel is flat fading to whole transmitted bandwidth signal frequency-selective channel for each sub-channel signal, equilibrium can be carried out respectively each subcarrier, has simplified receiver structure greatly.Because OFDM has availability of frequency spectrum height, balanced simple advantage, is very suitable for wired and wireless transmission at a high speed, has therefore obtained broad research.

Compare with single-carrier system, OFDM also has the shortcoming of self when having above advantage: very responsive to frequency shift (FS).In order to adopt the OFDM technology, carrier deviation is compared with subcarrier spacing, must be very little, otherwise the demodulation performance of OFDM will be subjected to very big influence.Yet because transceiver frequency stability and user's influences such as high-speed mobile, this requirement is difficult to be met, and must adopt advanced person's signal processing technology to estimate and compensates this frequency deviation.Simultaneously, the symbol timing of ofdm system must drop in the scope of Cyclic Prefix (CP) permission, otherwise will comprise the information of non-current code element in the FFT demodulation window, will cause the interference between code element.Therefore, effectively regularly also quite important to OFDM synchronously.

Utilize known information to analyze synchronously, as CP information, be current popular processing method, the article of delivering 97 years the 7th phases of IEEE Trans.SP as van de Beek J J etc. " MLEstimation of Time and Frequency Offset in OFDM System ".Equally also there is the document introduction to utilize additional information to carry out synchronous method, is 666237 on June 20th, 1996 at the application number of U. S. application as Schmidl etc., publication number is US's 5732113 " Timing andfrequency synchronization of OFDM signals " patent, this patent has proposed to utilize two synchronization preamble symbols to carry out symbol, the method of Frequency Synchronization, comprise two identical parts (frequency domain character is that the carrier wave of even number is 0 entirely) in first synchronizing symbol, use relevant method can obtain symbol synchronization information, utilize the phase information of synchronization point output that frequency deviation is proofreaied and correct, but frequency offset estimation range is less than a subcarrier spacing.Utilize the frequency deviation that estimates for the first time that two leading symbols are carried out frequency compensation and carry out the FFT computing then, utilize former and later two leading symbols integral multiple subcarrier frequency deviation to be estimated in the feature of frequency domain, the frequency deviation addition of twice estimation is promptly obtained final frequency offset estimation result, realized the synchronous of ofdm system.But because frequency offset estimation accuracy is not high, after channel equalization, must adds Phase Tracking, otherwise will cause the rotation of planisphere, cause the decreased performance of system.

Lai Nabo top grade on January 29th, 2002, the application number in Canada's application was 02806607.3, and publication number is that " frequency of OFDM receiver and the method for time synchronizedization " of CA 1531808A proposed to search for to obtain the method for synchronous of time and frequency in time, frequency bidimensional window.This method is at first determined the window of time synchronized and Frequency Synchronization, but this will utilize some prior informations, and such as the maximum frequency deviation scope, maximum delay scope etc. are carried out demodulation, equilibrium, judgement to received signal with each possible time and group of frequencies are incompatible then.When not having error or error very little synchronously, the error minimum in the restituted signal.This method has avoided using too much symbol to carry out having improved resource utilization ratio synchronously, yet this is to get in return with the huge operand of bidimensional search, and this method can not get the closed solutions of frequency deviation simultaneously.Draw thus, the defective that prior art exists awaits further improving and development.

Summary of the invention

Technical problem to be solved by this invention is to provide the synchronous method of realization in a kind of ofdm system, promptly provide in a kind of ofdm communication system the time one subsynchronous, frequency offset estimating, the time second synchronization, the scheme of time second synchronization reprocessing, thereby make the OFDM receiving system under with the cost of small system resources, realize symbol synchronization by low complexity algorithm, and added time synchronized post-processing module, just utilized another kind of method to replace the method for the existing CP of going, do not increase complexity, but but make the performance of receiver obtain obvious improvement.

For solving the problems of the technologies described above, the invention provides a kind of synchronous method that in ofdm system, realizes, comprise the steps:

(1) at the leading/synchronizing sequence of transmitting terminal structure, with the orthogonal frequency division multiplexing data symbols on the transmitting antenna with described leading/synchronizing sequence is respectively behind the framing and launch simultaneously;

(2) based on reception antenna, utilize Cyclic Prefix in the orthogonal frequency division multiplexing data symbols to carry out the relevant of significant character length, the correlated results that obtains carries out the time domain single treatment to the correlated series energy, obtains the set of optimal synchronisation point after subsynchronous;

(3) carry out the time domain frequency offset estimating based on leading/synchronizing sequence, the line frequency offset compensation of going forward side by side;

(4) in the range of convergence of the synchronous points of subsynchronous output of time one, the leading symbol that receiving terminal behind the compensate of frequency deviation is obtained and the leading symbol of described transmission carry out the time domain cross-correlation, it is surplus to utilize maximal peak point that circulating prefix-length is got, and obtains time second synchronization point;

(5) on the basis of time second synchronization point, utilize the signal post-processing after the time domain circular convolution characteristic of OFDM is carried out precise synchronization.

Wherein, step (1) described leading/synchronizing sequence, comprise Cyclic Prefix and PN sequence, targeting sequencing comprise one synchronously/leading symbol.

Described leading symbol is made of the identical PN sequence of length that repeats.

The length of described PN sequence is integral multiple/one of efficient orthogonal frequency division multiplexing data length.

Wherein, relevant can be represented by the formula of the described significant character length of step (2):

$b\left(k\right)=\underset{m=0}{\overset{P-1}{\mathrm{\Σ}}}r(m+k)r^{*}(m+k+N)$

Wherein, P represents the length of Cyclic Prefix symbol, and r is a receiving sequence.

The described time domain single treatment of step (2), choose greater than peaked half and less than peaked any one the number as decision threshold.

Wherein, described step (3) comprises, utilize first synchronous points in the time one minor synchronous point set, thresholding slip conjugate multiplication when carrying out, available leading symbol is carried out frequency offset estimating, and estimation range is at [N/2, N/2], wherein N is the number of times that leading symbol repeats on time domain, chooses as required.Wherein, described frequency offset estimating can be represented by the formula:

${\hat{f}}_{\mathrm{\Δ}}=\frac{\mathrm{angle}\left(z\right)}{2{\mathrm{\πN}}_S{T}_S}$

Wherein, T _SBe OFDM symbol period, N _SRepresent leading/the length of repetitive sequence on time domain that synchronizing sequence adopted.Z can be represented by the formula:

$z=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{r}_n{r}^{*}(n+N_S)=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{S}_n{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S}{S}^{*}(n+N_S)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}(n+N_S)T_S}$

${=e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}{N}_S{T}_S}\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|S_n\right|}^2$

Wherein, r is the targeting sequencing after subsynchronous through that receives, $S_n=r_n{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S},$ T _SBe the OFDM symbol period.

Wherein, the described leading symbol that receiving terminal behind the compensate of frequency deviation is obtained of step (4) and the leading symbol of described transmission carry out the time domain cross-correlation, can represent with following formula:

$c\left(k\right)=\underset{m=0}{\overset{N_{\mathrm{FFT}}-1}{\mathrm{\&Sigma;}}}{s}^{*}(k+m)r(k+m+N_{\mathrm{FFT}}),k={\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">k</figure-callout>}}_1,\&CenterDot;\&CenterDot;\&CenterDot;,$ $k_n$

Wherein, the targeting sequencing of s (k) for sending, N _FFTBe the size of OFDM symbol, the pilot frequency sequence of r (k) for carrying out receiving behind the frequency offset correction.

Wherein, the described signal post-processing of step (5) can be represented by the formula:

[Time_offset+N _cp/2：Time_offset+N _FFT-1，Time_offset：Time_offset+N _cp/2-1]

Wherein, Time_offset is the size that the estimated value of actual time delay adds the half cycles prefix, N _FFTBe the size of OFDM symbol, N _CpSize for Cyclic Prefix.

Of the present inventionly in ofdm system, realize synchronous method, owing to adopted the scheme of subsynchronous and time second synchronization and frequency offset estimating and reprocessing synchronously of time one, improved synchronization accuracy greatly, the received signal to noise ratio that can improve signal when satisfying the job requirement precision has promptly improved the performance of system, and do not expend too much system resource, can not increase the redundancy of system, even can improve the band efficiency of system in some cases, and operand is very little, is beneficial to very much Project Realization.

Description of drawings

Fig. 1 is according to realizing synchronous method flow schematic diagram in the described OFDM wireless system of the embodiment of the invention;

Fig. 2 is according to the schematic diagram of realizing a time method for synchronous in the synchronous method in the described OFDM wireless system of the embodiment of the invention;

Fig. 3 is according to the schematic diagram of realizing frequency deviation estimating method in the synchronous method in the described OFDM wireless system of the embodiment of the invention;

Fig. 4 is according to the schematic diagram of realizing time second synchronization method in the synchronous method in the described OFDM wireless system of the embodiment of the invention;

Fig. 5 is the structural representation according to time-domain symbol in the described OFDM wireless system of the embodiment of the invention;

Fig. 6 is the structural representation according to time synchronized point in the described OFDM wireless system of the embodiment of the invention.

Embodiment

Below in conjunction with accompanying drawing, specific embodiments of the invention are carried out comparatively detailed explanation.

With reference to figure 1, for realizing synchronous method flow schematic diagram in the described OFDM wireless system of the embodiment of the invention, detailed process is as follows:

Step 110: send at transmitting terminal leading/synchronizing sequence of structure and framing.

Transmitting terminal is constructed leading/synchronizing sequence when framing, its preferred version comprises Cyclic Prefix and PN sequence, and targeting sequencing can comprise at least one leading symbol, and its temporal signatures is that length is N _IThe PN sequence repeat I=N/N wherein I time _I, N is the sub-carrier number of OFDM.Send by transmitting antenna after the OFDM data symbol framing that should send.

Step 120: it is subsynchronous to carry out the time one in conjunction with CP information.

Described subsynchronous preferred version of time one is for carrying out the relevant of significant character length based on the CP in the receiving terminal OFDM symbol, and correlated results carries out normalized to the correlated series energy, be time Synchronous Processing, can choose greater than 0.5 times of maximum and less than peaked any one number and be decision threshold, can obtain the set of optimal synchronisation point after subsynchronous, as shown in Figure 2.

Step 130: utilize leading symbol to carry out integral multiple and decimal times time domain frequency offset estimating, the line frequency offset compensation of going forward side by side.

The described method preferred version that carries out frequency offset estimating based on targeting sequencing is first synchronous points of utilizing in the minor synchronous point set, and thresholding slip conjugate multiplication when carrying out utilizes maximum output to obtain the estimation of frequency deviation, the line frequency offset compensation of going forward side by side, as shown in Figure 3.

Step 140: utilize leading symbol to carry out the time second synchronization.

Described time second synchronization is included in the synchronous points range of convergence of a subsynchronous output, and leading symbol that receives behind the compensate of frequency deviation and transmission leading symbol are carried out the time domain cross-correlation, obtains time second synchronization point, as shown in Figure 4.

In the step 150: utilize the time second synchronization point that obtains, carry out synchronous reprocessing.Utilize the special construction of time second synchronization point and OFDM to adjust output sequence, as shown in Figure 5.

Wherein, in the synchronous method of described realization, described leading/synchronizing sequence comprises two parts: one is described Cyclic Prefix; The 2nd, the leading symbol that constitutes by the identical PN sequence of length, its length all is integral multiple/one of efficient orthogonal frequency division multiplexing data length, can be 1/2,1/4 or other value, for the integer frequency offset of the generation that guarantees ofdm system can not surpass estimation range, according to the number of times that repeats on the actual conditions design short preamble symbols time domain.Utilize leading symbol to carry out synchronously in the synchronous method of described realization, its output synchronously obtains a set that comprises best synchronous points, and relevant extreme value output back half choose synchronous points when promptly not having the OFDM symbol inter-block-interference, need to utilize the characteristic of OFDM Cyclic Prefix to carry out reprocessing after synchronous points is chosen.

Wherein, as described in step 110, the design of preamble structure as shown in Figure 6, number of sub carrier wave is 256.The k time sampling of received signal is output as r (k), is made of signal and receiver noise through channel.Sampled signal is cushioned, and buffer length is carried out relevant treatment to obtain time synchronizing information with symbol lengths of buffering signals time-delay then greater than an OFDM symbol.

Wherein, as described in step 120, with reference to figure 2, for realizing the schematic diagram of a time method for synchronous in the synchronous method in the described OFDM wireless system of the embodiment of the invention.Detailed process is as follows:

Step 210: sampled signal is cushioned, obtain buffering signals;

Step 220: buffering signals is carried out the N point postpone, be an effective OFDM symbol time time of delay, if corresponding sample frequency is the N gall nut carrier spacing, the delay sampling number is N so, otherwise will change;

Step 230: buffering signals and be delayed the signal that N order and arrived correlator by synchronous transport, carry out following operation:

$b\left(k\right)=\underset{m=0}{\overset{P-1}{\mathrm{\&Sigma;}}}r(m+k)r^{*}(m+k+N)$ Formula (1)

Wherein, P represents the length of CP symbol, and r is a receiving sequence.In k dropped on the length range of CP symbol constantly, this relevant output was very big, otherwise output is very little, and relevant peaks length is about the length of CP symbol.Above-mentioned relevant output can through type (2) iteration realize:

B (k+1)=b (k)-r (k) r ^*(k+N)+r (P+k) r ^*(P+k+N) formula (2)

Formula has reduced the related operation amount as can be known thus.

The output of described correlator is to the signal power normalization in the correlation time length, utilize normalization output and realize the time synchronized judgement via checkout gear, detection threshold is provided by system, this synchronous error is bigger, particularly under the Complex Channel situation, and because correlation length is limited, can not make full use of the power of whole symbol, therefore relevant affected by noise bigger.

Be not used for realizing synchronization decisions during present embodiment time one is subsynchronous, but possible synchronous points (point that relevant peaks is bigger) position is designated as { k ₁, k ₂..., k _nAs output, and via the time second synchronization of slip correlation preamble sequence finish final synchronously.

Wherein, as described in step 130, with reference to figure 3, for realizing the schematic diagram of frequency deviation estimating method in the synchronous method in the described OFDM wireless system of the embodiment of the invention.Detailed process is as follows:

Step 310: the sequence buffering to after subsynchronous is sampled, and obtains sampled data r (k);

Step 320: the sampled data r (k) of a subsynchronous postorder row buffering is postponed N sampled point backward;

Step 330: in a sliding window, carry out point-to-point time domain conjugate point and multiply each other, ask relevant according to formula (3) then:

$z=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{r}_n{r}^{*}(n+N_S)=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{S}_n{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S}{S}^{*}(n+N_S)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}(n+N_S)T_S}$

${=e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}{N}_S{T}_S}\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|S_n\right|}^2$ Formula (3)

Wherein, r is the targeting sequencing after subsynchronous through that receives, $S_n=r_n{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S},$ T _SBe OFDM symbol period, N _SRepresent leading/the length of repetitive sequence on time domain that synchronizing sequence adopted;

Step 340: correlated results is carried out normalized, and wherein, described normalization is meant carries out normalization on the amplitude to the value of obtaining of correlator, obtains the peak value after relevant.Shown in the accompanying drawing 3, estimated value/(2 π/N) be meant the normalization of carrying out the frequency offset estimating value, because the size of the skew that the normalization of an integer frequency offset causes is 2 π/N.

Step 350: estimate output frequency offset estimating value by (4) formula.

${\hat{f}}_{\mathrm{\&Delta;}}=\frac{\mathrm{angle}\left(z\right)}{2{\mathrm{\&pi;N}}_S{T}_S}$ Formula (4)

If obvious M _S=4, N _S=64, then the scope of frequency offset estimating is [2,2].Wherein, N _SRepresent leading/the length of repetitive sequence on time domain that synchronizing sequence adopted, M _SThe number of times that the expression repetitive sequence repeats.

Wherein, as described in step 140, with reference to figure 4, for realizing the schematic diagram of time second synchronization method in the synchronous method in the described OFDM wireless system of embodiment, detailed process is as follows:

Step 410: the sequence buffering is sampled, obtain sampled data r (k);

Step 420: with possible synchronous points set { k ₁, k ₂..., k _nAnd the buffering sampled data r (k) be input to cross-correlator, carry out operation suc as formula (5):

$c\left(k\right)=\underset{m=0}{\overset{N_{\mathrm{FFT}}-1}{\mathrm{\&Sigma;}}}{s}^{*}(k+m)r(k+m+N_{\mathrm{FFT}}),k={\mathrm{<figure-callout\; id="1"\; label="k"\; filenames="A20061008106600162.png,A20061008106600171.png"\; state="\{\{state\}\}">k</figure-callout>}}_1,\&CenterDot;\&CenterDot;\&CenterDot;$ $,k_n$ Formula (5)

Wherein, the targeting sequencing of s (k) for sending, N _FFTBe the size of OFDM symbol, the pilot frequency sequence of r (k) for carrying out receiving behind the frequency offset correction.Obviously, the correlated process of formula (5) can not realize with iteration, but its computing is only for gathering { k at a minor synchronous point ₁, k ₂..., k _nCarry out in the scope, so its operand is not very big.Simultaneously, because the time sampling sequence of received signal has experienced identical decline, therefore described time second synchronization is when accurate synchronous points, when formula (5) homophase addition, relevant peaks is more sharp-pointed, and maximum of points is a synchronous points, be subjected to the influence of long time delay for fear of received signal, cause synchronous points inaccurate, can get maximum of points CP length is got surplus method, to improve synchronous reliability.

Step 430: carry out synchronization decisions, export accurate synchronous points.

Make full use of leading structural design, make that the sync correlation peak output of this method is quite sharp-pointed, help synchronization decisions.Simultaneously, owing to utilized the signal energy of whole symbol, it is good therefore to work under little state of signal-to-noise.

Wherein, as described in step 150,, be the structural representation of time synchronized point in the described OFDM wireless system of the embodiment of the invention with reference to figure 5.

After the time second synchronization, general method of operation gets final product for directly removing CP.But this does not well utilize the characteristic of OFDM Cyclic Prefix, if utilize this characteristic, reduces the skew that synchronization delayed time causes, and improves the performance of receiver.Time_offset is the size that the estimated value of actual time delay adds half CP, actual consider to be this value characteristics that can utilize Cyclic Prefix can guaranteeing can not cause intersymbol interference, but well do not utilize the time delay estimated value well to reduce skew Effect on Performance.Therefore should take following post-processing approach after the time second synchronization, and then remove CP:

[Time_offset+N _Cp/ 2:Time_offset+N _FFT-1, Time_offset:Time_offset+N _Cp/ 2-1] formula (6)

In real system, two kinds of situations are arranged: the one, the frequency stability of working as transceiver is very high, and consider that the frequency deviation of this moment only was (to being a decimal after the subcarrier spacing normalization) of little several times when Doppler influenced the back frequency deviation value still less than the ofdm system subcarrier spacing.Frequency offset estimating only need be carried out fractional part of frequency offset and be estimated to get final product.

The 2nd, when the frequency stability of transceiver is not high, and consider when Doppler influences its value of back much larger than the ofdm system subcarrier spacing that in fact frequency deviation not only comprises little several times, but also comprises several integral multiples (being the integral multiple of subcarrier spacing).At this moment, leading symbol need be configured to shorter leading symbol, can enlarge frequency offset estimation range.

The maximum Doppler of the frequency stability of system's transceiver, system's support and subcarrier spacing are known, therefore be easy to learn that according to different system parameters designs frequency offset estimating is first kind of situation or second kind, carry out different processing according to different situations.

Can be drawn by the above, whole OFDM synchronizing process is divided into following step: construct leading and the framing emission; It is subsynchronous to carry out the time one in conjunction with CP; Utilize targeting sequencing to carry out frequency offset estimating; Utilize the leading time second synchronization of carrying out of being constructed; Reprocessing synchronously.The CP information of the subsynchronous OFDM of utilization of time one provides possible synchronous points scope for the time second synchronization, frequency offset estimating is partly carried out compensate of frequency deviation for the time second synchronization, time, the second synchronization part was at the designed leading relevant treatment of carrying out, obtain synchronizing information accurately, carry out synchronous reprocessing in conjunction with synchronous points, maximum possible obtain preferable received signal.

The method for synchronous of a kind of wireless OFDM disclosed in this invention system with traditional based on CP synchronously, compare based on the method for a plurality of leading symbols and multi-dimensional search and to have following characteristics: only need a leading symbol can realize the time synchronized of OFDM, for the system resource that time synchronized expends smaller; The realization synchronization accuracy is higher, its relevant peaks in time second synchronization output is quite sharp-pointed, output around maximum is all very little, be easy to adjudicate, and since time one subsynchronously the synchronous points scope is estimated, therefore time second synchronization operand is little, and can take synchronous reprocessing to improve the performance of system; Can be only realize in a big way frequency offset estimating with a short preamble symbols, this is based on, and method such as CP can't accomplish, and the Frequency Synchronization operand is very little; After the time second synchronization, the time synchronized reprocessing of carrying out does not increase operand, is equal to the original CP that goes substantially and handles, and has still reduced the skew of phase place, has improved the amplitude of received signal.

But should be understood that above-mentioned description at preferred embodiment of the present invention is comparatively concrete, can not therefore think the restriction to scope of patent protection of the present invention, scope of patent protection of the present invention should be as the criterion with claims.

Claims (10)

1. realize synchronous method in an ofdm system, it is characterized in that, comprise the steps:

(1) leading/synchronizing sequence of transmitting terminal structure, with the orthogonal frequency division multiplexing data symbols on the transmitting antenna with described leading/synchronizing sequence framing and launching simultaneously respectively;

(2) based on reception antenna, utilize Cyclic Prefix in the orthogonal frequency division multiplexing data symbols to carry out the relevant of significant character length, the correlated results that obtains carries out the time domain single treatment to the correlated series energy, obtains the set of optimal synchronisation point after subsynchronous;

(3) carry out the time domain frequency offset estimating based on leading/synchronizing sequence, the line frequency offset compensation of going forward side by side;

(4) in the range of convergence of the synchronous points of subsynchronous output of time one, the leading symbol that receiving terminal behind the compensate of frequency deviation is obtained and the leading symbol of described transmission carry out the time domain cross-correlation, it is surplus to utilize maximal peak point that circulating prefix-length is got, and obtains time second synchronization point;

(5) on the basis of time second synchronization point, utilize the signal post-processing after the time domain circular convolution characteristic of OFDM is carried out precise synchronization.

2. the method for claim 1 is characterized in that, step (1) described leading/synchronizing sequence, comprise Cyclic Prefix and PN sequence, targeting sequencing comprise one synchronously/leading symbol.

3. method as claimed in claim 2 is characterized in that, described leading symbol is made of the identical PN sequence of length that repeats.

4. as claim 2 or 3 described methods, it is characterized in that the length of described PN sequence is integral multiple/one of efficient orthogonal frequency division multiplexing data length.

5. the method for claim 1 is characterized in that, the relevant of the described significant character length of step (2) represented with following formula:

$b\left(k\right)=\underset{m=0}{\overset{P-1}{\mathrm{\&Sigma;}}}r(m+k)r^{*}(m+k+N)$

Wherein, P represents the length of Cyclic Prefix symbol, and r is a receiving sequence.

6. the method for claim 1 is characterized in that, the described time domain single treatment of step (2), choose greater than peaked half and less than peaked any one the number as decision threshold.

7. the method for claim 1, it is characterized in that, described step (3) comprises, utilize first synchronous points in the time one minor synchronous point set, thresholding slip conjugate multiplication is carried out frequency offset estimating with available leading symbol when carrying out, estimation range is at [N/2, N/2], wherein N is the number of times that leading symbol repeats on time domain, chooses according to the frequency deviation size that reality produces.

8. method as claimed in claim 7 is characterized in that, described frequency offset estimating is represented with following formula:

${\hat{f}}_{\mathrm{\&Delta;}}=\frac{\mathrm{angle}\left(z\right)}{2\mathrm{\&pi;}{N}_S{T}_S}$

Wherein, T _sBe OFDM symbol period, N _SRepresent leading/the length of repetitive sequence on time domain that synchronizing sequence adopted, z represents with following formula:

$z=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{r}_n{r}^{*}(n+N_S)=\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{S}_n{e}^{-j2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S}{S}^{*}(n+N_S)e^{-j2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}(n+N_S)T_S}$

$=e^{-j2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}{N}_S{T}_S}\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|S_n\right|}^2$

Wherein, r is the targeting sequencing after subsynchronous through that receives, $S_n=r_n{e}^{-j2\mathrm{\&pi;}{f}_{\mathrm{\&Delta;}}n{T}_S},$ T _SBe the OFDM symbol period.

9. the method for claim 1 is characterized in that, the described leading symbol that receiving terminal behind the compensate of frequency deviation is obtained of step (4) and the leading symbol of described transmission carry out the time domain cross-correlation, represent with following formula:

$c\left(k\right)=\underset{m=0}{\overset{N_{\mathrm{FFT}}-1}{\mathrm{\&Sigma;}}}{s}^{*}(k+m)r(k+m+N_{\mathrm{FFT}}),k=k_1,\&CenterDot;\&CenterDot;\&CenterDot;,k_n$

Wherein, the targeting sequencing of s (k) for sending, N _FFTBe the size of OFDM symbol, the pilot frequency sequence of r (k) for carrying out receiving behind the frequency offset correction.

10. the method for claim 1 is characterized in that, the described signal post-processing of step (5) is represented with following formula:

[Time_offset+N _cp/2：Time_offset+N _FFT-1，Time_offset：Time_offset+N _cp/2-1]

Wherein, Time_offset is the size that the estimated value of actual time delay adds the half cycles prefix, N _FFTBe the size of OFDM symbol, N _CpSize for Cyclic Prefix.

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